Archived posting to the Leica Users Group, 1999/05/27

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Subject: Re: [Leica] Re: A LEGITIMATE LEICA QUESTION
From: "Henning J. Wulff" <henningw@archiphoto.com>
Date: Thu, 27 May 1999 23:31:05 -0700

A little while ago, Roger Beamon wrote, almost in whole:

>On 26 May 99, Henning J. Wulff wrote, at least in part:
>
><snip>
>
>> Speaking of non-parallel surfaces on filters, I've had examples of this
>> from all manufacturers and suppliers (Hoya, Tiffen, H&H, Canon, Leica,
>> Heliopan, B+W, etc) except Nikon. Maybe I just haven't bought enough Nikon
>> filters, but I am up to about 40 from them.
>
>How does one check for surface parallelism? It's difficult for me to
>believe that filter surfaces could be so grossly aparallel [sic] as to
>be noticeable by the naked eye. Otherwise, some pretty

The easiest way, particularly with non-multicoated filters, is to lay one
on a matte (preferably dark) surface, and look at the reflection of a point
or sharply delineated ceiling light at about a 45 degree angle. Notice the
reflections off the back and off the front surfaces. Make sure you see
both. Now rotate the filter. If the two reflections change positions
relative to one another, the surfaces are not parallel. If the rate of
change of position is non-linear, one surface (or both) have curvatures in
them. This test is fairly sensitive. If you now take one of these filters
with non-parallel surfaces, and put it in or in front of a very highly
corrected apo or near apo tele lens, especially if it is more than 200mm in
focal length, _BAD_ things happen to the image quality. Very wide angle
lenses are also more sensitive to filter quality than lenses in the range
of 35mm to 90mm.

With multicoated filters the reflections off the back surface are sometimes
almost impossible to see. One hopes that since the manufacturers made the
effort to multicoat the filters, they also made the effort to make the
surfaces at least as parallel as decent float (window) glass. Unfortunately
this is not always true. To make sure the surfaces are smooth (which window
glass isn't, always..) the manufacturers grind or polish the surfaces,
obviously sometimes with misaligned machinery. Fortunately for us, bad
filters are becoming scarcer.

   *            Henning J. Wulff
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